Fractalkine (FKN) is a novel chemokine/mucin hybrid molecule expressed on the surface of activated endothelium which has dual functions as a chemokine and an endothelial adhesion molecule. Our preliminary studies have identified a unique role for FKN in the pathogenesis of transplant rejection. The objective of this proposal is to develop a detailed understanding of the proinflammatory actions of FKN and its receptor CX3CR1 in immune responses such as allograft rejection. We hypothesize that these actions are determined by regulated expression of FYN on endothelial cell surfaces and that one contribution of FKN is to directly promote cellular immune functions. We will test these hypotheses through the following specific aims: Specific Aim #1: To characterize the actions of FKN and CX3CR1 to regulate immune cell functions. In preliminary studies, we have identified potent actions of FKN to promote certain cellular immune functions. We will systematically examine the role of FKN in regulating the activities of two inflammatory cell populations that are known to express CX3CR1: NK cells and macrophages. We will focus on effector functions that are relevant to allograft rejection. Specific Aim #2: To identify the mechanisms that regulate FKN expression on endothelium in an inflammatory response. Our preliminary experiments have demonstrated novel regulation of endothelial expression of FKN by the lipid mediator thromboxane A2. These studies will define the molecular mechanisms of this regulation and will examine the relevance of this pathway in an ongoing inflammatory response. We posit that this may be a critical component of platelet-endothelial interactions during inflammation. Specific Aim #3: To define the role of FKN and CX3CR1 in the pathogenesis of allograft rejection. In these experiments, we will examine the contribution of FKN and CX3CR1 to allograft rejection in a series of experimental models. Using a combination of neutralizing antibodies and genetically altered mice, we will define the role of the FKN- CX3CR1 pathway in acute and chronic allograft injury. These studies will test whether the in vitro actions of FKN defined in previous aims are relevant to in vivo immune responses.